Package with solder regions aligned to recesses

ABSTRACT

A method includes forming a passivation layer over a portion of a metal pad, forming a polymer layer over the passivation layer, and exposing the polymer layer using a photolithography mask. The photolithography mask has an opaque portion, a transparent portion, and a partial transparent portion. The exposed polymer layer is developed to form an opening, wherein the metal pad is exposed through the opening. A Post-Passivation Interconnect (PPI) is formed over the polymer layer, wherein the PPI includes a portion extending into the opening to connect to the metal pad.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No. 13/926,981, entitled “Package With Solder Regions Aligned to Recesses,” filed on Jun. 25, 2013, which application is incorporated herein by reference.

BACKGROUND

In the integrated circuit manufacturing process, integrated circuit devices, such as transistors, are first formed at the surface of a semiconductor substrate in a wafer. An interconnect structure is then formed over the integrated circuit devices. Metal pads are formed over, and are electrically coupled to, the interconnect structure. A passivation layer and a first polymer layer are formed on the metal pads, with the metal pads exposed through the openings in the passivation layer and the first polymer layer. A Post-Passivation Interconnect (PPI) structure is then formed, which includes redistribution lines connected to the metal pads. A second polymer layer is then formed over the PPI. Under-Bump-Metallurgies (UBMs) are formed to extend into the openings in the second polymer layer, wherein the UBMs are electrically connected to the PPI. Solder balls are then placed over the UBMs and reflowed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1 through 12 are cross-sectional views of intermediate stages in the manufacturing of a wafer in accordance with some exemplary embodiments;

FIG. 13 illustrates a cross-sectional view of a portion of wafer in accordance with some exemplary embodiments, wherein a recess in a polymer layer has a curved bottom;

FIGS. 14 and 15 illustrate top views of a Post-Passivation Interconnect (PPI) structure in accordance with some exemplary embodiments; and

FIGS. 16A through 19 illustrate recesses extending into underlying polymer layers in accordance with various exemplary embodiments.

DETAILED DESCRIPTION

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative, and do not limit the scope of the disclosure.

A method for forming a Post-Passivation Interconnect (PPI) structure, an overlying solder region, and the resulting structure are provided in accordance with some embodiments. The intermediate stages of forming a respective wafer in accordance with some exemplary embodiments are illustrated. The variations of the embodiment are then discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

Referring to FIG. 1, wafer 100 is provided. Wafer 100 includes substrate 20, which may be a semiconductor substrate, such as a silicon substrate, although it may be formed of other semiconductor materials, such as silicon germanium, silicon carbon, gallium arsenide, or the like. Semiconductor devices 24, such as transistors, may be formed at the surface of substrate 20. Interconnect structure 22, which includes metal lines and vias 26 formed therein, is formed over substrate 20, and is electrically coupled to semiconductor devices 24. The metal lines and vias may be formed of copper or copper alloys, and may be formed using damascene processes. Interconnect structure 22 may include dielectric layers 25, which may include an Inter-Layer Dielectric (ILD, not shown) and Inter-Metal Dielectrics (IMDs, not shown). In alternative embodiments, wafer 100 is an interposer wafer or a package substrate, and is substantially free from integrated circuit devices including transistors, resistors, capacitors, inductors, and/or the like.

Metal pad 28 is formed over interconnect structure 22. Metal pad 28 may comprise aluminum (Al), copper (Cu), silver (Ag), gold (Au), nickel (Ni), tungsten (W), alloys thereof, and/or multi-layers thereof. Metal pad 28 may be electrically coupled to semiconductor devices 24, for example, through the underlying interconnect structure 22. Passivation layer 30 may be formed to cover the edge portions of metal pad 28. In some exemplary embodiments, passivation layer 30 comprises a silicon oxide layer and a silicon nitride layer over the silicon oxide layer, although other dielectric materials may be used. An opening is formed in passivation layer.

Polymer layer 32 is formed over passivation layer 30, wherein polymer layer 32 extends into the opening in passivation layer 30. Polymer layer 32 comprises a photo sensitive material, which may be a positive photo sensitive material or a negative photo sensitive material. For example, polymer layer 32 may comprise polyimide, polybenzoxazole (PBO), or the like. Polymer layer 32 is cured after it is applied on passivation layer 30. In some embodiments, polymer layer 32 has a planar surface in some embodiments.

In subsequent steps, polymer layer 32 is patterned. In some embodiments, partial-tone (also referred to as a partial tone) photolithography mask 200 is used to perform the exposure of polymer layer 32. Photolithography mask 200 includes portions 200A, 200B, and 200C. Portions 200A are opaque portions for blocking light 210, which is used for exposing polymer layer 32. Transparent portion 200B is a transparent portion, wherein light 210 is able to pass through with no degradation in the light intensity. Partial-blocking portions 200C, which are also partial-transparent portions, blocks a desirable percentage of the light 210, so that the light intensity LI2 of light 210′, which is the part of the light passing through partial-blocking portions 200C, is lower than the light intensity LI1 of light 210. In some embodiments, partial blocking portions 200C has light-passing rate LI2/LI1 greater than about 0.2, 0.3, 0.4, 0.5, 0.6, or any other positive value smaller than 1.0. Light-passing rate LI2/LI1 may also be smaller than about 0.8, 0.7, 0.6, 0.5, 0.4, or any other positive value greater than 0.0. As a comparison, opaque portions 200A may block greater than about 99 percent of the light (measured using light intensity), and transparent portion 200B may allow more than 99 percent of the light (measured using light intensity) to pass through. It is realized that depending on the material of photolithography mask 200, the light-passing rate and light-blocking rate of portions 200A and 200B may vary slightly.

In some embodiments, photolithography mask 200 includes transparent substrate 202, which may be formed of glass, quartz, or the like. An opaque layer 204 is formed on substrate 202. Opaque layer 204 is patterned, and the remaining portions of opaque layer 204 form the opaque portions 200A. Some portions of substrate 202 do not have opaque layer 204 thereon, and hence the respective portions are transparent portions 200B or partial transparent portions 200C. In some exemplary embodiments, opaque layer 204 comprises chromium, although other materials that are efficient in blocking light may be used. It is appreciated that whether a layer is transparent or not is related to the material and its thickness. Accordingly, opaque layer 204 is thick enough (with thickness greater than 1,000 Å, for example) to block light. Partial transparent portions 200C may comprise partially transparent layer 206, which is formed of Molybdenum Silicide (MoSi) or other types of materials, and the light-passing ratio LI2/LI1 may be adjusted through the adjustment of the material and/or the thickness of partial transparent layer 206. For example, MoSi layer 206 may have a thickness between about 800 Å and about 1,600 Å.

After the exposure of polymer layer 32 through lithography mask 200, lithography mask 200 is removed. Polymer layer 32 is then developed, and the exposed portions are removed. The resulting structure is shown in FIG. 2. In FIG. 2, lithography mask 200 is illustrated to show the relative positions of some features, although lithography mask 200 is not over wafer 100 at this time. Accordingly, lithography mask 200 is illustrated using dashed lines. As shown in FIG. 2, a portion of polymer layer 32 is aligned to transparent portion 200B when polymer layer 32 is exposed, and hence after the development of polymer layer 32, this portion is removed to form opening 29, through which metal pad 28 is exposed. The portions of polymer layer 32 aligned to opaque portions remain un-removed. The portion of polymer layer 32 aligned to partial transparent portion 200C is partially exposed, which means that although this portion is exposed, the light amount is not enough to cause this portion of polymer layer 32 to be fully removed in the development step. As a result, after the development of polymer layer 32, a top layer of the partially exposed portion of polymer layer 32 is removed, while the bottom layer of the partially exposed portion of polymer layer 32 is not removed. Recess 34 is hence formed in polymer layer 32, which has depth D1 smaller than thickness T1 of polymer layer 32. In some embodiment, ratio D1/T1 is between about 0.2 and 0.8, or between 0.2 and 0.5, although other values may be used. Depth D1 may be greater than about 1.6 μm to allow the subsequently applied solder ball to stay stably in place.

It is appreciated that in the example used in FIGS. 1 and 2, polymer layer 32 is a positive photo material, so that the exposed portions are removed in development step, and un-exposed portions are not removed. Opening 29 and recess 34 may also be formed using the same concept when polymer layer 32 is a negative photo material. In which embodiments, however, the positions of the opaque portions 200A and transparent portion(s) 200B (FIG. 1) are swapped, while the location of partial transparent portion(s) 200C remains.

Next, as shown in FIG. 3, seed layer 40 is formed over polymer layer 32. Seed layer 40 may include layers 40A and 40B. Layer 40A may be a titanium layer, a titanium nitride layer, a tantalum layer, a tantalum nitride layer, or the like. The materials of layer 40B may include copper or copper alloys. In some embodiments, seed layer 40 is formed using physical vapor deposition, while other applicable methods may also be used.

FIG. 4 illustrates the formation of mask 46. In some embodiments, mask 46 is formed of a photo resist, and hence is alternatively referred to as photo resist 46 throughout the description, although other materials may be used. A portion of seed layer 40 is exposed through opening 48 in mask 46. Next, as shown in FIG. 5, a plating step is performed to form PPI 50 in opening 48. PPI 50 may be formed of copper or copper alloys, and may include PPI line 50A, PPI pad 50B, and PPI via 50D. A top view of exemplary PPI 50 is shown in FIG. 14.

After the formation of PPI 50, mask 46 is removed. Next, the exposed portions of seed layer 40 that were previously covered by photo resist 46 are removed by etching, while the portions of seed layer 40 covered by PPI 50 remain un-removed. The resulting structure is shown in FIG. 6. Throughout the description, the remaining portions of seed layer 40 are considered as parts of PPI 50.

As shown in FIG. 6, PPI pad 50B comprises a portion in recess 34 (FIG. 2) of polymer layer 32. PPI pad 50B may further comprise an additional portion outside recess 34 and over polymer layer 32. PPI pad 50B may be a conformal layer, and hence the portions of PPI pad 50B in recess 34, the portions of PPI pad 50B over polymer layer 32, and possibly the slanted portions on the sidewalls of recesses 34 have substantially the same thickness. Accordingly, recess 34′ is formed in PPI pad 50B, and may have depth D1′ similar to depth D1 in FIG. 2.

In some exemplary embodiments, as shown in FIG. 7, metal finish 56 is formed on PPI pad 50B. In alternative embodiments, metal finish 56 is not formed, and metal bump 58 (FIG. 8) is attached directly on PPI pad 50B. Accordingly, metal finish 56 is illustrated using dashed lines to indicate that they may or may not exist. In the embodiments in which metal finish 56 are formed, metal finish 56 may be an Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), which includes a nickel layer, a palladium layer on the nickel layer, and a gold layer on the palladium layer. The gold layer may be formed using immersion plating. In other embodiments, metal finish 56 may be formed of other finish materials and using other methods, including, but not limited to, Electroless Nickel Immersion Gold (ENIG), Direct Immersion Gold (DIG), or the like. In some embodiments, metal finish 56 is formed in the opening in photo resist 52. After metal finish 56 is formed, photo resist 52 is removed. In some embodiments, metal bump 58 is a solder ball, and hence is referred to as solder ball 58 throughout the description, although it may also be other types of metal bumps such as copper balls.

FIG. 8 illustrates the application of flux 54 on PPI pad 50B (or on metal finish 56, if any). Next, stencil 57 is placed on wafer 100, and solder ball 58 is dropped in opening 60 of stencil 57. Opening 60 is aligned to PPI pad 50B, so that solder ball 58 may be accurately placed on PPI pad 50B or metal finish 56. Next, stencil 57 is removed, and a reflow is performed to reflow solder ball 58, and hence solder ball 58 is joined to, and in contact with, PPI pad 50B or metal finish 56. The resulting structure is shown in FIG. 9.

During the reflow of solder ball 58, since solder ball 58 is self-aligned to recess 34′ (FIG. 6) and recess 34 (FIG. 2), it is difficult for solder ball 58 to shift in position, and hence the position of solder ball 58 is accurately aligned to the desirable position, which is defined by the opening 60 in stencil 57 (FIG. 8).

Next, as shown in FIG. 10, liquid molding compound 62 is applied on wafer 100, wherein solder ball 58 may be submerged under liquid molding compound 62. Referring to FIG. 11, release film 64 is applied on liquid molding compound 62. A pressure is applied, as shown by arrows 63. Release film 64 is formed of a soft material, so that a portion of solder ball 58 is pressed into release film 64. Furthermore, release film 64 pushes some of liquid molding compound 62 away from the top surface of wafer 100, and the bottom surface of release film 64 is lower than the top end of solder ball 58. With release film 64 remaining being pushed against solder ball 58 and liquid molding compound 62, a curing is performed to cure and solidify liquid molding compound 62. After the solidification of molding compound 62, the top end of solder ball 58 is lower than the top surface of molding compound 62.

Release film 64 is then peeled off from molding compound 62, which is now in a solid form. The resulting structure is shown in FIG. 12. The molding compound residue remaining on the top surface of solder ball 58 is etched. In the resulting structure, molding compound 62 is formed with a portion of solder ball 58 buried therein. The top end of solder ball 58 may be higher than the top surface of molding compound 62.

FIG. 15 illustrates a top view of molding compound 62 and the underlying PPI 50 and recess 34. FIG. 15 illustrates PPI line 50A, PPI pad 50B, and transition portion 50C connecting PPI line 50A to PPI pad 50B. Transition portion 30C may have a gradually changed width, so that the connection between PPI line 50A and PPI pad 50B is not abrupt. Recess 34′/34 may be aligned to the center of PPI pad 50B.

Referring back to FIG. 12, in subsequently steps, wafer 100 is sawed into dies, wherein each of the dies is bonded to another package component such as package substrate, interposer, Printed Circuit Board (PCB), or the like to form a package. Solder ball 58 will also be joined with the conductive feature on the other package component. Since the position of solder ball 58 is accurately defined due to the formation of recess 34 in polymer 32 (FIG. 2), the misalignment between solder ball 58 and the conductive feature of the package component is reduced.

FIG. 13 illustrates a possible profile of PPI pad 50B and metal finish 56, if any. In some embodiments, the recess 34 (filled by PPI pad 50B) in polymer layer 32 has a curved and smooth bottom surface. As a result, metal finish 56 also has curved and smooth top and bottom surfaces. The curved surface may be caused by the optical effects.

FIGS. 16A through 19 illustrate the cross-sectional views and top views of PPI pad 50B in accordance with alternative embodiments, wherein more than one recess 34 correspond to the same PPI pad 50B. The number of recesses 34 corresponding to each of PPI pad 50B may be any positive integer such as 2, 3, 4, 5, or the like. For example, FIG. 16A illustrates a cross-sectional view of some embodiments, in which there are two recesses 34 formed in polymer layer 32, wherein PPI pad 50B extends into recesses 34. FIG. 16B illustrates a top view of recesses 34 corresponding to what are shown in FIG. 16A. FIG. 16C further illustrates molding material 62.

In FIGS. 14, 15, and 16C, recesses 34 have round top view shapes. FIG. 17 illustrates a plurality of rectangular recesses 34/34′, which may have square top-view shapes. FIG. 18 illustrates that recesses 34/34′ have strip-shapes, for example, with length L1 greater than two times width W1. In FIG. 18, the lengthwise direction of recesses 34 are parallel to the lengthwise direction of the respective connecting PPI line 50A. FIG. 19 illustrates an embodiment similar to what is shown in FIG. 18, except that the lengthwise direction of recesses 34 is perpendicular to the lengthwise direction of the respective connecting PPI line 50A.

In the embodiments of the present disclosure, by using a partial-tone photolithography mask to pattern the polymer layer, on which the PPI is formed, the polymer layer may have a recess formed therein, wherein the PPI pad extends into the recess. The PPI pad hence also has a recess. As a result, the shift in the position of the solder ball placed thereon is substantially eliminated. The embodiments have improved accuracy in the alignment of the solder ball, while no extra manufacturing cost is incurred.

In accordance with some embodiments, a method includes forming a passivation layer over a portion of a metal pad, forming a polymer layer over the passivation layer, and exposing the polymer layer using a photolithography mask. The photolithography mask has an opaque portion, a transparent portion, and a partial transparent portion. The exposed polymer layer is developed to form an opening, wherein the metal pad is exposed through the opening. A Post-Passivation Interconnect (PPI) is formed over the polymer layer, wherein the PPI includes a portion extending into the opening to connect to the metal pad.

In accordance with other embodiments, a method includes forming a passivation layer over a portion of a metal pad, forming a polymer layer over the passivation layer, and patterning the polymer layer to form an opening and a recess. The opening penetrates through the polymer layer, wherein the metal pad is exposed through the opening. The recess extends into but does not penetrate through the polymer layer. A PPI is formed over the polymer layer, wherein the PPI includes a PPI via extending into the opening to connect to the metal pad, and a PPI pad comprising a portion in the recess. The method further includes placing a solder ball over the PPI pad, and performing a reflow on the solder ball. The solder ball is aligned to the recess, and is electrically coupled to the PPI pad.

In accordance with yet other embodiments, a device includes a substrate, a metal pad over the substrate, a passivation layer over a portion of the metal pad, a polymer layer over the passivation layer, and a PPI. The PPI includes a PPI via penetrating through the polymer layer to connect to the metal pad, and a PPI pad electrically coupled to the PPI via. The PPI pad includes a portion extending into a recess of the polymer layer, with the recess extending from a top surface of the polymer to an intermediate level of the polymer layer. A solder region is overlying and electrically coupled to the PPI pad, wherein the solder region is directly over the PPI pad.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. A method comprising: forming a passivation layer over a portion of a metal pad; forming a polymer layer over the passivation layer; light-exposing the polymer layer using a photolithography mask, wherein the photolithography mask comprises an opaque portion, a transparent portion, and a partially transparent portion; developing the polymer layer to form an opening, wherein the metal pad is revealed through the opening, and a recess extends partially into the polymer layer; and forming a Post-Passivation Interconnect (PPI) over the polymer layer, wherein the PPI comprises a first portion extending into the opening to connect to the metal pad, and a second portion extending into the recess, wherein the first portion and the second portion are portions of a continuous region.
 2. The method of claim 1, wherein the photolithography mask comprises a transparent layer, and an opaque layer and a partially transparent layer on the transparent layer, and wherein: the opaque layer and a first portion of the transparent layer in combination form the opaque portion of the photolithography mask; the partially transparent layer and a second portion of the transparent layer in combination form the partially transparent portion of the photolithography mask; and a third portion of the transparent layer forms the transparent portion.
 3. The method of claim 1, wherein in the light-exposing and the developing, a portion of the polymer layer corresponding to the transparent portion is removed to form the opening, and an upper portion of the polymer layer corresponding to the partially transparent portion is removed to form the recess, and a lower portion of the polymer layer corresponding to the partially transparent portion is unremoved.
 4. The method of claim 1, wherein the recess has a continuously curved bottom.
 5. The method of claim 1, wherein in a same process the recess is formed, a plurality of additional recesses is formed in the polymer layer, and wherein the second portion of the PPI extends into the recess and the plurality of additional recesses.
 6. The method of claim 5 further comprising forming a solder region overlapping and contacting the second portion of the PPI, wherein the second portion of the PPI has a top surface contacting the solder region, and a bottom surface contacting the polymer layer.
 7. The method of claim 1, wherein the recess extends partially into the polymer layer.
 8. A method comprising: forming a passivation layer over a portion of a metal pad; forming a polymer layer comprising: an opening penetrating through the polymer layer, wherein the metal pad is revealed through the opening; and a recess extending partially into the polymer layer; forming a conductive feature over the polymer layer, wherein the conductive feature comprises: a via portion extending into the opening to connect to the metal pad; and a pad portion comprising a portion in the recess; forming a solder region over and electrically coupling to the pad portion, wherein a portion of the solder region extends into a recess of the pad portion; and applying an encapsulating material on the solder region so that a lower portion of the solder region is encapsulated in the encapsulating material, and an upper portion of the solder region is exposed.
 9. The method of claim 8, wherein the forming the polymer layer comprises: applying the polymer layer; and forming the opening and the recess in a common process.
 10. The method of claim 9, wherein the common process comprises a common light-exposure to the polymer layer and a common development of the polymer layer.
 11. The method of claim 10, wherein the common light-exposure is performed using a lithography mask having at least three portions with three transparency levels.
 12. The method of claim 8, wherein the encapsulating material is in physical contact with the solder region, and the encapsulating material forms a continuously curved interface with the solder region.
 13. The method of claim 8, wherein the recess has a depth, and the polymer layer has a thickness, and wherein a ratio of the depth to the thickness is between about 0.2 and about 0.5.
 14. The method of claim 8, wherein the solder region fully fills all remaining portions of the recess unoccupied by the portion of the pad portion.
 15. A method comprising: forming a polymer layer over a metal pad; performing a light-exposure on the polymer layer using a photolithography mask, wherein the photolithography mask comprises first portion, a second portion being more transparent than the first portion, and a third portion being more transparent than the second portion; developing the polymer layer to form a recess extending into the polymer layer, wherein a portion of the polymer layer remains directly underlying the recess after the polymer layer is developed, and the recessed portion of the polymer layer is exposed by a light passing through the second portion of the photolithography mask; and forming a conductive feature over the polymer layer, wherein the conductive feature comprises a first portion extending into the recess, and second portion encircling the first portion, wherein the second portion is outside of the recess.
 16. The method of claim 15, wherein in the developing, an additional portion of the polymer layer exposed through the third portion of the photolithography mask is removed to expose the metal pad, and the conductive feature has a portion contacting the metal pad.
 17. The method of claim 15, wherein in the developing, an additional portion of the polymer layer aligned to the first portion of the photolithography mask during the light-exposure is not removed.
 18. The method of claim 15, wherein the second portion of the photolithography mask has a light-passing rate between about 20 percent and about 80 percent.
 19. The method of claim 15, wherein the recess has a bottom at an intermediate level between a top surface and a bottom surface of the polymer layer.
 20. The method of claim 15 further comprising: placing a solder ball on the first portion of the conductive feature; and performing a reflow to join the solder ball to the first portion of the conductive feature. 